Spare, comparatively ancient, fuel tank (9)
I believe the answer is:
gasholder
'fuel tank' is the definition.
(I've seen this before)
'spare comparatively ancient' is the wordplay.
'spare' becomes 'gash' (I've seen this before).
'comparatively ancient' becomes 'older' ('old' can be similar in meaning to 'ancient').
'gash'+'older'='GASHOLDER'
(Other definitions for gasholder that I've seen before include "fuel storage facility" , "Heard logs (anag) - fuel store" , "Storage tank" , "Fuel tank" .)